Once you have used a referral code I think it stays that way. There is no way to transfer referrals. Why do you want to change it?
So many questions later have we narrowed this down to being a request by @Ngoc to have HIS/HER Referal code that they give to friends changed to a different one? Presumably because it is a confusing code likely including 0-O and or 1-l-L and or 5-S in it? From the day that Referal codes were changed from being 10 digit phone numbers to these 6 digit Referal codes many have asked “What were they thinking” when they didn’t simply remove these confusing letters/numbers from being assigned. But...it is what it is.
In the case that you successfully used a code but charged your mind and want to use a different one.
People tried to do it and they were told the only way you can do that is to open brand new account and use the correct code.
One person however managed to do it as that person was tricked into using different code, it was a sales person who entered the wrong (possibly their own) code instead of the one provided by the customer. They were persistent and they succeeded.
So maybe you could give it a try.
In the case where you forgot to use it or it was invalid so that you didn't get h your referral credit (hence your account is not linked to any referral code) then moderators will help you and add it manually